Protective circuit for protection against over voltage for a can- bus transceiver

ABSTRACT

The invention relates to a protective circuit for a CAN-bus transceiver that is designed voltagewise for a first vehicle electric system, said transceiver being operated in a second vehicle electric system with a voltage that is several times higher than the voltage of the first vehicle electric system, comprising two diodes arranged between the bus terminal leads of the transceivers whose cathodes are connected to one another and to which a predetermined potential is applied, in addition to a limiting resistor mounted between each of the bus terminal leads of the transceiver and the bus line assigned thereto and two current mirror circuits for maintaining the require voltage level in the bus lines.

The invention relates to a protective circuit for protection againstover-voltage for a CAN bus transceiver designed in voltage terms for afirst vehicle electrical system, said transceiver being operated in asecond vehicle electrical system, in particular in a motorized vehicleelectrical system, having a voltage several times that of the firstvehicle electrical system either alone or in a two-voltage vehicleelectrical system with the first and the second vehicle electricalsystem, according to the features of claim 1.

The introduction of motorized vehicle electrical systems having voltagesof 14V+42V or, as the case may be, only 42V has been the subject ofdiscussion for some time and is now imminent. The greatest obstacle toemploying the electronic components used in the 14V vehicle electricalsystem in the 42V vehicle electrical system is the lack of short-circuitstrength of said electronic components in association with 42V.

Whereas a short-circuit strength in the presence of 14-18V (permanently)and in the presence of 32-36V (transiently) has previously been adequatein the 14V vehicle electrical system (Vbat1=12V), a short-circuitstrength of 58V (permanently) and of up to 70V (transiently) is requiredin the 42V vehicle electrical system (Vbat2=36V).

Existing ASIC circuits having been optimized also in terms of dielectricstrength for the 14V vehicle electrical system, their direct use in the42V vehicle electrical system is for the most part not possible. Thatcan as a rule only be achieved by using another, dielectrically strongersemiconductor technology.

A change of technology of said type is as a rule tantamount to aredesign of the respective ASIC circuit, with the concomitant costthereof amounting to millions and with a development period of severalyears.

Alternative approaches are necessary in order to provide pilot seriesproduction models for the 42V vehicle electrical system with suitableelectronic components. It is possible, in particular for input andoutput functions having low driver power, to find protective circuitsthat will perform splitting in the event of short-circuiting to 42Vvehicle electrical system voltages. If said protective circuits arestructured discretely, the result together with the original functionalmodules will be assemblies that are 42V-compatible.

Extensive investigations indicate an urgent need for 42V-compatiblecommunication interfaces. This applies particularly to the CAN bustransceiver, CAN having become the standard in automobile applicationsand now being employed in virtually every engine management andtransmission control system.

A successful discrete circuit design can also serve as a basis forsubsequent integration.

A circuit arrangement for a network terminating unit for coupling anddecoupling useful signals and feeding terminating equipment at four-wireinterfaces in digitally controlled communication networks is known fromDE 197 33 250 A1, the feed circuit therein being protected against briefovercurrent by current-limiting means or, as the case may be, againstcontinuous overcurrent by means of a feed deactivator having atime-controlled feed activator, and said circuit arrangement having anactive line driver for achieving the necessary transmission level on thelines.

Disclosed in DE 43 27 035 A1 is an arrangement for the bi-directionaltransmission of data on a two-wire BUS system, which arrangementoperates in the single-wire mode in the presence of a multiplicity ofline faults, thereby preventing power losses also in the event ofshort-circuiting to a high operating voltage.

The object of the invention is to provide a protective circuit that issuitable for the CAN bus transceiver, technically easy to implement, andcapable of being integrated, and which will enable a transceiverdesigned for the 14V vehicle electrical system to be used also in the42V vehicle electrical system.

Said object is achieved according to the invention by means of aprotective circuit having the features of claim 1.

Advantageous developments of the invention are indicated in thesub-claims.

The invention encompasses the technical principle of inserting acurrent-limiting resistor into each of the two lines of the CAN bus forthe purpose of limiting the short-circuit currents and restoring thetransmitter's then reduced driver power, taking specific EMC factors(common mode signal) into consideration, by means of an additionalcircuit that will be deactivated in the event of short-circuiting to 42V(self-protection).

An exemplary embodiment according to the invention is explained in moredetail below with reference to a schematic drawing.

FIG. 1 is a general diagram of a known CAN bus with a transceiver,

FIG. 2 is a block diagram of a CAN bus transceiver,

FIG. 3 a shows the ideal signals on the lines of the CAN bus,

FIG. 3 b shows the real signals on the lines of the CAN bus, and

FIG. 4 is a diagram of the protective circuit according to theinvention.

FIG. 1 is a general diagram of a known version of the CAN bus fordifferential data transmission in a 14V vehicle electrical system, saidbus having a first bus line (HI) and a second bus line (LO) generallyembodied as a twisted pair. The HI line is highlighted in bold inFIG. 1. At one end of the CAN bus line is a transceiver TC thatcommunicates with a control device (microcomputer, controller etc.);connected to the other end is a device G which is to be driven via theCAN bus and which is itself connected to the CAN bus via a transceiver(not shown). Further devices G (and transceivers) can be connected atany point in the CAN bus. Each transceiver of a further device Grequires a protective circuit according to the invention againstshort-circuits in the 42V vehicle electrical system. A Philips PCA82C250whose data is contained in the data sheet “Philips semiconductorsPCA82C250 CAN controller interface, product specification” dated Jan.13, 2000, is used, for instance, as the transceiver TC for a high-speedversion.

The line impedance is 120Ω, for example; the CAN bus is accordinglyterminated on each side by two resistors R (each rated 60Ω) connected inseries between the lines HI and LO and by a grounded capacitor C (rated100 nF) situated therebetween. The thus obtained low-level impedance toground helps in suppressing (EMC) common mode signals.

FIG. 2 is a block diagram of a CAN bus transceiver TC. It comprises atransmitter TM (transmitter module) and a receiver RC (receiver module).A high-level resistor network for setting the direct voltage operatingpoint is also integrated.

Said resistor network consists, for instance, of a resistor RT1connected between the positive terminal Vcc of the supply voltage of thetransceiver TC and the LO line of the CAN bus, a resistor RT2 connectedbetween the HI and LO line, and a resistor RT3 connected between the HIline and ground GND. This is a possible circuit for generating a directvoltage level of 2.5V. RT1 and RT3 here have the same, high-resistancevalue (for example 100 kΩ each), while RT2 has a lower resistance (forexample 5 kΩ). Through this arrangement the voltage on the HI line isslightly less than on the LO line, that situation being highlydesirable. With this circuit dimensioning the differential inputimpedance measurable on the transceiver terminals TCHI and TCLO isapproximately 5 kΩ.

A somewhat more detailed diagram of the transceiver TC can be seen inthe block diagram in FIG. 1 of the already mentioned Philips data sheetof the PCAS2C250 CAN controller interface.

As shown in FIG. 3 a, two level statuses can be generated on the buslines HI and LO:

-   -   a) both lines are applied to a direct voltage potential        V(HI)=V(LO)=+2.5V. This status corresponds to the “recessive” L        level of the control signal st;    -   b) a direct voltage potential V(HI)=3.5V (2.5V+1V) is applied to        the line HI and a direct voltage potential V(LO)=1.5V (2.5V−1V)        is applied to the line LO. This status corresponds to the        “dominant” H level of the control signal st.

The purpose of this is to ensure that the total voltage V(HI)+V(LO)=5Vof both lines is constant at all times, thereby minimizing theoccurrence of high-frequency noise radiation (EMC).

As the additional potentials (±1V) are not activated and deactivated atexactly the same time in known transceiver embodiments, voltage peaks(what are termed “spikes”) occur in the summation signal duringswitching which give rise to undesired, high-frequency interferencesignal radiation, see FIG. 3 b. This effect is counteracted by insertinga CAN bus choke DR between the transceiver TC and the lines HI and LO ofthe CAN bus, see FIG. 4.

Said choke DR acts like a transformer balancing out the differences inthe signal curves between the lines so that the signal shapes can bemade to approach the ideal. This minimizes the “spikes” and reduces theEMC noise radiation.

The transmitter TM is protected against short-circuiting both to ground(0V), to negative voltages (ground displacements, negative transientvoltages), and to battery voltage Vbat1 (to 14-18V permanently and to32-36V transiently). This measure is, however, ineffectual in the caseof short-circuiting to 42V because the breakdown voltage of thetransistors and protective diodes is far exceeded. The result in thiscase is excessive current flow and destructive overheating of the ASICcircuit.

The same protective measures apply to the receiver RC as to thetransmitter.

The fatal effect in the case of short-circuiting in the 42V vehicleelectrical system (58V permanently and up to 70V transiently) is due tothe high value of the voltage and the resulting currents. A protectivecircuit ought not to restrict the transceiver's functioning in any way,but on the other hand it ought reliably to keep harmful voltage levelsaway from the transceiver terminals.

FIG. 4 shows a circuit according to the invention by means of which atransceiver TC that is designed for a 14V vehicle electrical systemVbat1 and which is operated in a two-voltage vehicle electrical systemVbat1+Vbat2 is reliably protected against short-circuiting in the 42Vvehicle electrical system (˜60V permanently and ˜70V transiently). Thisis achieved by fixing the voltages on the transceiver terminals TCHI,TCLO to the battery voltage Vbat1 (+14V) and by limiting the faultcurrent via limiting resistors inserted into the bus lines, whichresistors must be dimensioned in such a way (each 1 kΩ/1 W, for example)that the receiver function of the transceiver TC is not impaired.

The transmitter then, however, being decoupled from the CAN bus by saidlimiting resistors, operation requires an additional circuit that willensure the direct voltage level of 2.5V is maintained on the bus linesbut which itself needs to be protected against short-circuiting in the42V vehicle electrical system (60/70V).

Shown in FIG. 4 is the CAN bus as in FIG. 1. The transceiver TC islocated at one end of the CAN bus (although only said transceiver'stransmitter TM is shown here); the CAN bus lines HI and LO, highlightedby dashed lines, are again terminated on both sides by the two resistorsR connected in series between the lines HI and LO and by the groundedcapacitor C therebetween. For the sake of clarity the lines are notshown twisted; nor are the devices and transceivers requiring to beconnected indicated, although the already mentioned choke DR between thetransceiver TC and the CAN bus lines is shown.

A drive source μC (microcomputer, controller etc.) supplies the controlsignal st for the transmitting operation of the transceiver TC. Thelimiting resistors R3 and R4 are inserted as series resistors betweenthe outputs of the transceiver TC and the bus lines HI and LO. Locatedbetween the two bus terminals (HI and LO) of the transceiver TC are twodiodes D3 and D3′ whose cathodes are connected to each other and to apredefined potential, for example to that of the first vehicleelectrical system voltage Vbat1 (+12V) whose negative terminal isapplied to ground GND.

If there is only one 42V vehicle electrical system Vbat2, the cathodesof the two diodes D3 and D3′ can be applied to an existing potential orto a suitably dimensioned Zener diode. The value of the predefinedpotential P or, as the case may be, the value of the breakdown voltageVz of the Zener diode can be within a range between the supply voltageVcc of the transceiver TC and the vehicle electrical system voltage forwhich the transceiver TC is designed (in this case Vbat1).

The transceiver terminals TCHI, TCLO having been decoupled from the CANbus via the resistors R3 and R4, the transceiver is no longer able togenerate the necessary voltage levels V(HI)=3.5V and V(LO)=1.5V on thebus lines HI, LO.

For this reason two current-mirror circuits Q1-Q2 and Q3-Q4 are providedwhich perform this function. To generate the reference current for thefirst current-mirror circuit (Q1-Q2) and second current-mirror circuit(Q3-Q4), a resistor (R6) and a third transistor (Q5) are insertedbetween the transistors (Q11 and Q3) of the two current-mirror circuits(Q1-Q2, Q3-Q4), which transistors are arranged in series between thepositive terminal (+Vcc) of the supply voltage (Vcc) of the transceiver(TC) and ground (GND).

Transistor Q2, which together with the transistor Q1 forms the firstcurrent-mirror circuit, is connected to the positive terminal +Vcc ofthe supply voltage via a resistor R2 and to the bus line HI via a diodeD1 (in the conducting direction toward the bus line HI; as protectionagainst reverse voltage).

Transistor Q4, which together with the transistor Q3 forms the secondcurrent-mirror circuit, is connected to ground GND via a resistor R8 andto the bus line LO via a diode D2 (in the conducting direction away fromthe bus line LO; as protection against reverse voltage).

Both current-mirror circuits must be designed for an output current ofthis type in such a way that when driven by the transceiver TC they willbe able to generate the necessary voltage excursion on the CAN bus of+1V on the line HI and of −1V on the line LO (=2V peak-to-peak).

Both current-mirror circuits Q1-Q2, Q3-Q4 are activated and deactivatedsynchronously with the control signal st of the transceiver TC via thethird transistor Q5.

A series arrangement comprising a Zener diode D4 and two resistors R9and R10 is located between the bus line LO and ground GND. Theconnection point of the two resistors is connected to the base of atransistor Q6 whose emitter is applied to ground GND and whose collectoris connected to the base of the third transistor Q5. The twocurrent-mirror circuits Q1Q2, Q3-Q4 will be deactivated by said circuitarrangement as soon as the voltage on one of the CAN bus lines exceeds avalue of, for example, the voltage (+12V) of the first vehicleelectrical system Vbat1.

The corresponding diode D3, D3′ will become conductive ifshort-circuiting occurs on one of the CAN bus lines in the 42V vehicleelectrical system (up to 60/70V on the HI or LO line). The current willbe limited by the limiting resistors R3, R4 to, for example, 30 mA,which is why they must be designed for a higher power level, for example1 kΩ/1 W, as already mentioned. The transceiver outputs will be limitedthrough this measure to a voltage Vbat1+0.7V increased by the voltagedrop on the diode D3, D3′. The transceiver is internally protectedagainst a voltage of this type.

The transceiver TC remains de-energized while data is being received andin the recessive phase; in the dominant phase the current will belimited to approximately 40 mA.

The current-mirror circuits Q1-Q2 and Q3-Q4 are protected by means ofthe two diodes D1, D2 against both polarity reversal andshort-circuiting in the 42V vehicle electrical system on one of the CANbus lines. Both current-mirror circuits are additionally protected bythe transistor Q6, which will deactivate the two current-mirror circuitsas soon as the voltage on one of the CAN bus lines exceeds a value of,for example, 12V.

The receiver function of the transceiver TC, which is to say when thetransceiver of a device G (FIG. 1) transmits to the receiver of thetransceiver TC, will not be impaired by the described measures. If theleast favorable value (5 kΩ) is assumed for the differential inputresistance, then the result, together with the limiting resistors R3,R4, will be a voltage divider which, although attenuating the bus signaltoward the receiver (from ±1V to approximately ±0.7V), will neverthelessdeliver a value conforming to the specifications of the PCA82C250transceiver for instance employed.

The result for the transceiver protected in the described manner is asfollows:

The protective circuit will reliably protect the transceiver againstshort-circuiting (permanently at least up to 60V and transiently 70V) onthe bus lines,

the protective circuit will be intrinsically safe and easy to implementusing standard components;

the protective circuit's design concept will be suitable for integratingin an ASIC circuit; compliance will be ensured with the relevantspecification parameters of the transceiver (for example of thePCA82C250 and CAN bus).

The output signals of the transmitter TM modified according to theinvention are very symmetric so that it will probably be possible todispense with the CAN bus choke DR without exceeding the radiated noiselimits. This means a further cost saving.

The time up until when a fully integrated solution is available can bebridged using the protective circuit according to the invention.

1-7. (canceled)
 8. A protective circuit for protecting a CAN bustransceiver against overvoltage, wherein the CAN bus transceiver isconfigured, in voltage terms, for a first vehicle electrical system, andthe CAN bus transceiver is operated in a second vehicle electricalsystem having a voltage multiple times a voltage of the first vehicleelectrical system either alone or in a two-voltage vehicle electricalsystem with the first vehicle electrical system and the second vehicleelectrical system, the CAN bus transceiver having two bus terminals forconnection to respective bus lines and a supply voltage source of thetransceiver with a positive terminal and a negative terminal, theprotective circuit comprising: two diodes connected between thebusterminals of the transceiver, said diodes having cathodes connectedto one another and to a predefined potential; a limiting resistorconnected between each bus terminal of the transceiver and the bus lineassigned thereto; and a first current-mirror circuit connected betweenthe positive terminal of said supply voltage source of the transceiverand the first bus line, and a second current-mirror circuit connectedbetween the second bus line and ground, for restoring voltage levelsreduced by said limiting resistors on the bus lines.
 9. The protectivecircuit according to claim 8, wherein a value of the predefinedpotential is within a range between the supply voltage of thetransceiver and the vehicle electrical system voltage for which thetransceiver is configured.
 10. The protective circuit according to claim8, wherein the predefined potential corresponds to a breakdown voltageof a Zener diode having a value within a range between the supplyvoltage of the transceiver and the vehicle electrical system voltage forwhich the transceiver is configured.
 11. The protective circuitaccording to claim 8, which comprises, for generating a referencecurrent for said first current-mirror circuit and said secondcurrent-mirror circuit, a resistor and a third transistor insertedbetween transistors of said first and second current-mirror circuits,wherein said transistors are connected in series between the positiveterminal of said supply voltage source and ground.
 12. The protectivecircuit according to claim 11, wherein said current-mirror circuits areselectively activated and deactivated via said third transistor by wayof a control signal controlling a transmitting operation of thetransceiver.
 13. The protective circuit according to claim 8, whichcomprises a series circuit including a Zener diode and two resistorsconnected between one bus line and ground, a further transistor having abase connected to a node between said resistors, an emitter connected toground, and a collector connected to a base of said third transistor,wherein said two current-mirror circuits are deactivated when a voltageon one of the CAN bus lines exceeds a specific value determined by wayof said series circuit of said Zener diode and said two resistors. 14.In combination with a device having a plurality of CAN bus transceiversconnected to a CAN bus, a plurality of protective circuits according toclaim 8 each assigned and connected to a respective CAN bus transceiverfor over-voltage protection.